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Understanding Unsprung Weight (Hub Motors)

Every few months a discussion comes up about hub motors and their pros and cons, and as you know if you’ve been reading this site for a while, I’m a big fan of the hub motors idea and I’d like them to be awesome. The unsprung weight issue is a killer, though, and what’s bugging me is that a lot of people post opinions that basically say, it’s not that bad, it’s over-blown, and even it doesn’t matter. Some will even tell you it’s an advantage. I beg to differ.

The key here, and please pay attention, is that you better know what you’re getting into before you go down this road. Pun intended.

Like anything else we build there’s good and bad, but if you’re listening to advice that’s, at best, cognitivly dissonant, you may be in for some very expensive disappointment, not to mention, potentially hazardous riding. To really understand the issues, let’s understand what unsprung weight is, and how it affects handling.

What is unsprung weight? Basically, the weight that’s south of the suspension. The tire, the wheels, brakes, spokes, and lower part of the front forks, as well as a lot of the swingarm, sprockets and chain are all mass, or weight, which has to be supported by the springs and shocks. OK now, some basic physics. What’s that law – inertia? “… the resistance of any physical object to any change in its state of motion”? The more mass you have, the more inertia you get. The bottom line, for suspension systems, is that the less unsprung weight you have, the less inertia, so the more responsive the suspended parts are going to be.

Keeping in mind the sole point of suspension is to keep the tires in contact with the road, then you can take that a step further. The lower your unsprung weight, the more road contact your tires will have under any conditions, and the more control you’ll have over the handling of the bike.

Not only is it a simple concept, but it’s something that’s been the focus of all handling technology – cars as well as motorcycles – for well over nine decades. Ninety, frikkin, years. Yes. Here you go, from the Wiki:

The term ‘unsprung mass’ was coined by the mathematician Albert Healey of the Dunlop tyre company. He presented one of the first lectures taking a rigid analytical approach to suspension design, ‘The Tyre as a part of the Suspension System’ to the Institution of Automobile Engineers in November 1924.[1] This lecture was published as a 100 page paper.[2]

Unsprung weight has quite a few implications as well. If you’re running a lighter assembly your suspension can be lighter too. With a more responsive suspension you can run a softer suspension, and it’s argued you can get a more comfortable ride. Your spinning mass is lighter, too, which means more efficiency and better range.

There they have a nice little pro and con breakdown, but the one that’s most to this point is #2 on the “cons” list – Hub motors can snap dropouts. This is a pretty interesting observation, because when you bolt a big hub motor into a motorcycle, nobody is talking about the stresses you’re putting on the swingarm. I’m not saying you’re going to snap it, but you’re certainly giving it a lot more stress than it was designed for, and in different ways. Every bit of swingarm design is about rigidity, and by putting a 100lbs-plus addition to your mass in there, you’re not doing the swingarm any favors.

So there are some really basic, and frankly, undeniable conclusions about the hub motor’s impact on a bike’s handling.

Unsprung weight adversely affects handling, by decreasing the responsiveness of the suspension.

Unsprung weight also increases the weight of the suspension system required to counteract the increased mass.

As the bike’s speed increases the affects of unsprung weight increase.

As the road surface becomes more uneven, the affects of unsprung weight increase.

I think most of us can agree with these statements. Certainly 90 years of suspension technology will corroborate them. Most of the comments about the handling degradation of unsprung weight come as ideology and value judgements, however. Here’s a sample:

They add unsprung weight, but that’s a minor tradeoff for the space savings.

Handling might take a small hit, but it worked well enough for Catavolt to become 2012 75 class champs.

The unsprung weight isn’t a problem for 99% of the riders !

Good of these folks to decide who-all us 99% are and what we want in a motorcycle… but whatever, it’s the kind of advice you see on forums.

So, yes, for a rider who’s mainly commuting, not riding at speed (read: over the speed limit), or who’s just poking around the neighborhood a hub motor may be a great option. A lot of scooters run hub motors, but they’re generally limited to a below-50mph top speed, and if you’ve ever gone faster on a scooter I’d wager you won’t want to again soon. Scariest shit I’ve seen on two wheels, doing stuff like that. If, however, you want to ride your bike fast, hit the twisties, ride on roads that aren’t in California (that is, they have bumps) like here in New England, are used to modern sport bike handling, or even envision yourself, one day, when you least expect it, hitting a curve a little too hot on not so great a line and seeing road chop coming up on the apex, then I’d seriously give hub motors some second thoughts.

I promised I’d comment on Catavolt’s use of the double Enertrac hub motor if I did this post. Well, here you go.

The Catavolt bike won it’s class out of a field of three bikes, if memory serves – not ot depreciate the win, but still. It was running on a completely flat, even track. It can hardly be held up as an example of good handling with huge unsprung weight against a fair field of competition. The notion that the weight somehow adds to the rear traction simply defies everything we understand about suspension, sorry. The description of the wheel being “glued to the track” works fine until you hit a rough section, and the adaptation of rider style is just that – a workaround – and goes more to support my point than anything else said.

I don’t want this to appear a rant. I get that people love their hub motors, and they do have huge advantages, but please, let’s not try to redefine physics or pass off personal preferences and value judgements as fact. If you’re looking in to hub motors, some simple advice – know what your tradeoffs are, and decide if you can live with them. Hell. People ride 1200cc 600lb+ motorcycles with hardtails and love them, right? (…for short trips, at any rate.)

Here’s the conclusion of one poster, and it’s an example of the decisions you have to make for yourself:

It’s good to hear that the unsprung weight is a surmountable issue – it’s still not a setup that I’d opt for but my handling expectations may be higher than average, or perhaps it’s due to the nature of our roads over here where routes are far more often bendy than straight, and rarely all that smooth, so handling and suspension come to the forefront.

That is a fair appraisal – If handling and suspension are at the forefront, then you have to consider unsprung weight to be an issue.

Thanks Ted!! Love your objective in helping us do it as yourselfers with some of our desicions and just outright knowledge and help. I’m going hub cause there’s a charging item that needs to be brought forward. Be safe!

A good analysis but, considering the multiple acknowledgments of the benefits of hub motors, I’d like to have seen some discussion of potential paths to develop technologies to ameliorate the problems. For example, increasing suspension stiffness and then suspending the seat and rider. (Which alludes to a small beef with one statement; there are secondare considerations in suspension design, not just one) Another might be more sophisticated control of spring and damper rates.

On a suspension bike, a hub motor definitely increases unsprung weight and adversely affects handling. It also makes for a harsh ride that can just about shake the fillings out of your teeth!

But here’s the thing: on a bike without discrete suspension, the “suspension” has traditionally been mostly in the tires and spokes. Sure, the old steel frames and forks, seat post and handle bars flex a little, too, but mostly it’s the tires and spokes that provide most of the cushioning over bumps, which means that a hub motor is actually sprung weight.

Here’s the bummer: the hub motor is still rotating mass, which adversely affects acceleration and steering.

Here’s the other bummer: hub motors typically require really short, stiff spokes, so if you want a cushy ride, you have to drop the tire pressure, which also makes for mushy handling, and can increase the risk of blow-outs.